CULTURE MEDIA PREPARATION
1 Aim
To precisely formulate, sterilize, and dispense commonly used microbiological culture media—Nutrient Agar (NA), Luria-Bertani (LB) Broth, and Potato Dextrose Agar (PDA)—for the in vitro cultivation of bacteria and fungi.
2 Principle
Microorganisms require specific biochemical environments to thrive. Artificial culture media are designed to provide essential macronutrients (C, N, P, S), micronutrients, vitamins, and a stable pH. Media can be liquid (broth) for high-yield suspension cultures, or solid (agar) for isolating pure colonies.
Targeted Media Selection:
- Nutrient Agar (NA): A complex, general-purpose medium that supports the growth of a wide range of non-fastidious bacteria.
- LB Broth (Luria-Bertani): A nutritionally rich medium specifically optimized for the rapid growth of Escherichia coli, widely used in molecular cloning.
- Potato Dextrose Agar (PDA): A carbohydrate-rich, slightly acidic medium (pH 5.6) designed to favor the growth of fungi and yeasts while suppressing bacterial growth.
3 Media Recipes (For 1 Liter Volume)
4 Procedure Step-by-Step
Step A. Weighing and Mixing
- For PDA Only: Boil 200g of diced potatoes in 500 ml distilled water for 30 mins. Filter through muslin cloth to collect the potato infusion. Discard the solid mass.
- Weigh out all specific ingredients carefully.
- In a large Erlenmeyer flask (at least 1.5L capacity to prevent boiling over), add the dry ingredients (and potato infusion if making PDA).
- Add distilled water to bring the final volume to exactly 1000 ml.
Step B. Heating and Dissolving
- Place the flask on a magnetic stirrer/hot plate. Drop in a magnetic flea.
- Heat until boiling. Agar requires heat to melt; the cloudy suspension will turn completely clear once the agar dissolves (~85°C).
- Check and adjust the pH using 1N NaOH or 1N HCl using a pH meter or pH strips.
Step C. Autoclave Sterilization
- Plug the flask tightly with non-absorbent cotton and wrap the top with aluminum foil to prevent steam from soaking the cotton.
- Place inside an Autoclave.
- Sterilize at 121°C (250°F) at 15 psi pressure for 15–20 minutes.
Step D. Pouring the Plates (For Agar Media)
- Carefully remove the flask using heat-resistant gloves.
- Transfer to a Laminar Airflow Cabinet.
- Allow the media to cool to 45°C – 50°C. (It should feel warm but tolerable against the inside of your wrist).
- Aseptically pour ~20-25 ml of media into sterile Petri dishes.
- Leave the lids slightly ajar inside the running laminar flow hood to allow steam to escape and prevent condensation.
- Once solidified, store the plates inverted at 4°C.
5. Observation & Results
| Medium | Physical State | Visual Appearance |
|---|---|---|
| Nutrient Agar | Solid (Gel) | Light yellow, translucent. |
| LB Broth | Liquid | Clear, pale golden-yellow. |
| PDA | Solid (Gel) | Light brown, slightly opaque. |
6. Precautions & Troubleshooting
| Action / Observation | Reasoning & Solution |
|---|---|
| Plates are stored INVERTED | Condensation naturally forms on the lid. If stored right-side up, water drops will fall onto the agar, causing bacterial colonies to mix and smear together. |
| Agar fails to solidify | Either you forgot to add the agar powder (making broth instead), or the pH was highly acidic before autoclaving, which breaks down agar molecules (acid hydrolysis). |
| Media boils over in the autoclave | You filled the flask too high. Never fill a flask more than 50% to 60% of its total capacity when autoclaving. |
🧠Interactive Viva Quiz
Test your knowledge! Click on the questions below to reveal the correct answers.
1. Why do we use Agar instead of Gelatin to solidify media?
✅ Answer: Agar is thermally stable and biologically inert.
Gelatin melts at a low temperature (around 28°C), meaning it would turn to liquid in a 37°C incubator! Furthermore, many bacteria produce gelatinase enzymes that digest gelatin, turning the plate into liquid. Agar (derived from red algae) melts at 85°C, solidifies at 40°C, and is not degraded by the vast majority of microbes.
2. Why is an Autoclave set specifically to 121°C at 15 psi?
✅ Answer: To completely destroy bacterial endospores.
Boiling water at normal atmospheric pressure (100°C) kills vegetative cells but cannot kill highly resistant bacterial endospores (like Bacillus and Clostridium). By increasing the pressure to 15 psi, steam can reach 121°C, which is the exact thermal death point required to completely obliterate endospores, ensuring absolute sterility.
3. Why is PDA slightly acidic (pH 5.6)?
✅ Answer: To selectively inhibit bacterial growth.
Fungi and yeasts tolerate acidic environments much better than most bacteria. Maintaining PDA at pH 5.6 allows the fungi to grow vigorously while naturally suppressing unwanted bacterial contamination.
4. What makes LB Broth a "Complex" or "Undefined" medium?
✅ Answer: The exact chemical formula is unknown.
Because LB broth contains "Yeast Extract" and "Tryptone" (digested milk proteins), it contains a massive, unquantified variety of amino acids, vitamins, and minerals. We don't know the exact molarity of every single carbon or nitrogen atom, making it an undefined medium (unlike a synthetic medium made from pure salts and glucose).
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